Akatsuki at Venus: The First Year of Scientific Operation

A.Ikeshita

Editorial Board (March 2020)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

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Solid Earth Science

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Space Science

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Earth, Planets and Space
Nakamura et al. Earth, Planets and Space (2018) 70:144
https://doi.org/10.1186/s40623-018-0916-3

Open Access

PREFACESpecial issue “Akatsuki at Venus: The FirstYear of Scientific Operation”
Masato Nakamura1*, Dmitri Titov2, Kevin McGouldrick3, Pierre Drossart4, Jean‑Loup Bertaux5,6 and Huixin Liu7of instruments can monitor the dynamics of the wholeatmosphere in three dimensions.
The second attempt of the orbit insertion using attitude control thrusters in 2015 was perfectly successful.
The first scientific discovery was made just 3 h after thearrival: the longwave infrared camera captured a planetary-scale atmospheric gravity wave that was stationary
with respect to the surface topography. Starting withthis discovery, many other scientific results have beenaccomplished.
This issue presents initial results of the Akatsuki mission. Iwagami et al. (2018) reported the initial resultsfrom Akatsuki/1-μm camera (IR1). More than 600 dayside and 150 night-side images have been obtainedsince the beginning of regular operation on April 2016.
The night-side images are less numerous due to limitations related to the light scattered from the bright dayside. Satoh et al. (2017) reported the performance of
Akatsuki/2-μm camera (IR2), and the results obtainedwith IR2 camera from December 2015 through November 2016. A total of 3091 images of Venus (1420 dayside
images at 2.02 μm and 1671 night-side images at 1.735,
2.26, and 2.32 μm) were acquired in this period. Additionally, 159 images, including those of stars for calibration and dark images for the evaluation of dark levels,
were obtained. Yamazaki et al. (2018) described the initial results from Akatsuki/Ultraviolet Imager (UVI) at
283 nm and 365 nm. The UV images provide the spatial
distribution of SO2 and the unknown absorber at thecloud tops and characterize the cloud-top morphologies and haze properties. Nominal sequential images
with 2-h intervals are used to understand the dynamics
of the Venusian atmosphere by deriving the wind velocity
field from measured motion vectors at the cloud tops, aswell as the mass transportation of UV absorbers. Fukuhara et al. (2017) described the calibration of the Akatsuki/longwave infrared camera (LIR). The brightness

The JAXA Venus explorer, which had been developed inJapan since 2001, was launched in 2010. The spacecraft
was named “Akatsuki” after the Japanese word meaning
dawn. Akatsuki was inserted into the Venus orbit andbegan its observation program in 2015, after 5 years wandering around the sun due to the failure of orbit insertionin 2010. Nakamura et al. (2016) described the orbit insertion as well as some initial results from the IR1, UVI, and
LIR cameras.
In the twentieth century, well before the start of Akatsuki mission, the former Soviet Union and the USA had
explored Venus and elucidated basic characteristics suchas atmospheric temperature, pressure, composition, windspeed, surface topography and the plasma environment.
However, the physical processes leading to the present
state were still unclear. Among them, the Akatsuki mission focused on the study of atmospheric dynamics as amajor goal of Venus exploration in the early twenty-first
century. The Venusian atmosphere encircles the planet
from east to west at all latitudes at a speed much fasterthan the solid planet. This wind system, called the superrotation, is one of the biggest mysteries of planetarymeteorology and should have a major effect on Venus’
environment. Akatsuki is aimed at a comprehensive
survey of the meteorological processes and a quantitative evaluation of the momentum transport sustaining
the super-rotation. For this purpose, five cameras imaging the planet at different wavelengths are installed on
the spacecraft to visualize atmospheric motions at different altitudes. Combined with a radio occultation
instrument that probes the vertical structure, this suite*Correspondence: mnakamur@stp.isas.jaxa.jp
1
Institute of Space and Astronautical Science, Japan Aerospace
Exploration Agency, Sagamihara, Japan
Full list of author information is available at the end of the article© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

1

Akatsuki at Venus: The First Year of Scientific Operation
Nakamura et al. Earth, Planets and Space (2018) 70:144

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the atmosphere and are thus a key diagnostic tool whenquantitative dynamical analysis is not feasible due toinsufficient information.
In this special issue, you will find the history of thespacecraft development, the design of the observation
instruments, the data processing procedure and the initial scientific results. We hope this special issue willfamiliarize readers with the outline of the JAXA Akatsuki
mission that opened up a new era of Venus exploration.
Akatsuki follow-up Venus missions are planned by somespace agencies and are discussed by Glaze et al. (2018).

temperature derived from LIR images contained an unexpected bias related not to natural phenomena but to athermal condition of the instrument. Deep-space images
were acquired at different baffle temperatures, and a reference table for eliminating the bias from images was
prepared. Takahashi et al. (2018) described the lightning
search using the Akatsuki/lightning and airglow camera(LAC). LAC was designed to observe the light curve of
possible flashes at a sufficiently high sampling rate to discriminate lightning from other sources and thereby perform a more definitive search for optical emissions. It was
confirmed that the operational high voltage was achieved
and that the triggering system functions correctly. LAC
lightning search observations are planned to continue forseveral years.
Imamura et al. (2017) reported the initial results of
the radio occultation experiments revealing the Venus
atmosphere structure. The physical quantities retrieved
include the pressure, the temperature, the H2SO4 vapor
density, and the ionospheric electron density and theirvariations.
Archiving of the results of these observations is important. Ogohara et al. (2017) gave an overview of the data
products. The level-2 images include calibrated radiancesand geometry information. The level-3 data are globalgrid data in a regular longitude–latitude coordinate system. The method of correcting the boresight pointing ofeach camera by fitting an ellipse to the observed Venusian limb is also described.
Three numerical modelling and theoretical works areincluded in this special issue. Horinouchi et al. (2018)studied the cloud-top wind field using images takenby the Ultraviolet Imager (UVI) at the wavelengths of
365 nm (unidentified absorber) and 283 nm (SO2). These
two wavelengths yield slightly different wind velocities,
suggesting a difference in the altitude of the cloud features. The local-time dependence and an asymmetry withrespect to the equator were observed. The geographic
distribution of the zonal wind reported previously (Bertaux et al. 2016) was not seen in the data. McGouldrick(2017) studied the effects of variation in the coalescence
efficiency of the Venus cloud particles on the structureof cloud using a one-dimensional cloud model. Specifically, they explored the consequence of allowing the coalescence efficiency of supercooled sulfuric acid in theupper clouds to tend to zero. The most significant result
is the appearance of thick clouds of small particles near
the transition between upper and middle clouds. Limaye
et al. (2018) summarized the characteristics of Venus
clouds seen in the multi-wavelength images taken by
Akatsuki. The images reveal new and puzzling morphology of the complex cloud cover. The cloud morphologies provide some clues to the processes occurring in

Authors’ contributions
All authors read and approved the final manuscript.
Author details
1
Institute of Space and Astronautical Science, Japan Aerospace Exploration
Agency, Sagamihara, Japan. 2 ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk,
The Netherlands. 3 Laboratory for Atmospheric and Space Physics, Universityof Colorado Boulder, 3665 Discovery Dr., Boulder, CO 80303, USA. 4 LESIA,
Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris
Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France.
5
LATMOS/UVSQ, Guyancourt, France. 6 IKI/RAS, Moscow, Russia. 7 Departmentof Earth and Planetary Science, Faculty of Science, Kyushu University, Fukuoka,
Japan.
Competing interests
The authors declare they don’t have any competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in pub‑lished maps and institutional affiliations.
Received: 14 August 2018 Accepted: 29 August 2018

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Akatsuki at Venus: The First Year of Scientific Operation

FRONTIER LETTER

Open Access

AKATSUKI returns to Venus
Masato Nakamura*, Takeshi Imamura, Nobuaki Ishii, Takumi Abe, Yasuhiro Kawakatsu, Chikako Hirose, Takehiko Satoh,
Makoto Suzuki, Munetaka Ueno, Atsushi Yamazaki, Naomoto Iwagami, Shigeto Watanabe, Makoto Taguchi, Tetsuya Fukuhara,
Yukihiro Takahashi, Manabu Yamada, Masataka Imai, Shoko Ohtsuki, Kazunori Uemizu, George L. Hashimoto, Masahiro Takagi,
Yoshihisa Matsuda, Kazunori Ogohara, Naoki Sato, Yasumasa Kasaba, Toru Kouyama, Naru Hirata, Ryosuke Nakamura,
Yukio Yamamoto, Takeshi Horinouchi, Masaru Yamamoto, Yoshi-Yuki Hayashi, Hiroki Kashimura, Ko-ichiro Sugiyama,
Takeshi Sakanoi, Hiroki Ando, Shin-ya Murakami, Takao M. Sato, Seiko Takagi, Kensuke Nakajima, Javier Peralta,
Yeon Joo Lee, Junichi Nakatsuka, Tsutomu Ichikawa, Kozaburo Inoue, Tomoaki Toda, Hiroyuki Toyota,
Sumitaka Tachikawa, Shinichiro Narita, Tomoko Hayashiyama, Akiko Hasegawa and Yukio Kamata
Earth, Planets and Space 2016, 68:75 DOI:10.1186/s40623-016-0457-6
Received: 5 January 2016, Accepted: 29 April 2016, Published: 6 May 2016

Abstract
AKATSUKI is the Japanese Venus Climate Orbiter that was designed to investigate the climate system of Venus.
The orbiter was launched on May 21, 2010, and it reached Venus on December 7, 2010. Thrust was applied by
the orbital maneuver engine in an attempt to put AKATSUKI into a westward equatorial orbit around Venus
with a 30-h orbital period. However, this operation failed because of a malfunction in the propulsion system.
After this failure, the spacecraft orbited the Sun for 5 years. On December 7, 2015, AKATSUKI once again
approached Venus and the Venus orbit insertion was successful, whereby a westward equatorial orbit with
apoapsis of ~440,000 km and orbital period of 14 days was initiated. Now that AKATSUKI’s long journey toVenus has ended, it will provide scientific data on the Venusian climate system for two or more years. For the
purpose of both decreasing the apoapsis altitude and avoiding a long eclipse during the orbit, a trim maneuver
was performed at the first periapsis. The apoapsis altitude is now ~360,000 km with a periapsis altitude of
1000–8000 km, and the period is 10 days and 12 h. In this paper, we describe the details of the Venus orbit
insertion-revenge 1 (VOI-R1) and the new orbit, the expected scientific information to be obtained at this orbit,
and the Venus images captured by the onboard 1-µm infrared camera, ultraviolet imager, and long-wave
infrared camera 2 h after the successful initiation of the VOI-R1.

Keywords: Venus, Atmosphere, Meteorology, Exploration, AKATSUKI

Fig. 1*Corresponding author: Masato Nakamura, nakamura.masato@jaxa.jp

FRONTIER LETTER

Open Access

Performance of Akatsuki/IR2 in Venus orbit: the first year
Takehiko Satoh*, Takao M. Sato, Masato Nakamura, Yasumasa Kasaba, Munetaka Ueno, Makoto Suzuki,
George L. Hashimoto, Takeshi Horinouchi, Takeshi Imamura, Atsushi Yamazaki, Takayuki Enomoto,
Yuri Sakurai, Kosuke Takami, Kenta Sawai, Takashi Nakakushi, Takumi Abe, Nobuaki Ishii, Chikako Hirose,
Naru Hirata, Manabu Yamada, Shin-ya Murakami, Yukio Yamamoto, Tetsuya Fukuhara, Kazunori Ogohara,
Hiroki Ando, Ko-ichiro Sugiyama, Hiroki Kashimura and Shoko Ohtsuki
Earth, Planets and Space 2017, 69:154 DOI:10.1186/s40623-017-0736-x
Received: 29 June 2017, Accepted: 23 October 2017, Published: 3 November 2017

Abstract
The first year (December 2015 to November 2016) of IR2 after Akatsuki’s successful insertion to an elongated elliptical orbit
around Venus is reported with performance evaluation and results of data acquisition. The single-stage Stirling-cycle cryo-coolerof IR2 has been operated with various driving voltages to achieve the best possible cooling under the given thermal
environment. A total of 3091 images of Venus (1420 dayside images at 2.02 μm and 1671 night-side images at 1.735, 2.26, and2.32 μm) were acquired in this period. Additionally, 159 images, including images of stars for calibration and dark images for theevaluation of noise levels, were captured. Low-frequency flat images (not available in prelaunch calibration data) have been constructed using the images of Venus acquired fromnear the pericenter to establish the procedure to correct for the IR2 flat-field response. It wasnoticed that multiple reflections of infrared light in the PtSi detector caused a weak but
extended tail of the point-spread function (PSF), contaminating the night-side disk of Venus
with light from the much brighter dayside crescent. This necessitated the construction of anempirical PSF to remove this contamination and also to improve the dayside data bydeconvolution, and this work is also discussed. Detailed astrometry is performed on star-field
images in the H-band (1.65 μm), hereby confirming that the geometrical distortion of IR2
images is negligible.

Keywords: Venus, Atmosphere, Orbiter, Akatsuki, Near-infrared, Window, Cryo-cooler,
Dynamics, Aerosols

Fig. 1*Corresponding author: Takehiko Satoh, satoh@stp.isas.jaxa.jp
4

Earth, Planets and Space

FULL PAPER

Open Access

Initial performance of the radio occultation experiment in the Venus
orbiter mission Akatsuki
Takeshi Imamura*, Hiroki Ando, Silvia Tellmann, Martin Pätzold, Bernd Häusler, Atsushi Yamazaki, Takao M. Sato,
Katsuyuki Noguchi, Yoshifumi Futaana, Janusz Oschlisniok, Sanjay Limaye, R. K. Choudhary, Yasuhiro Murata, Hiroshi Takeuchi,
Chikako Hirose, Tsutomu Ichikawa, Tomoaki Toda, Atsushi Tomiki, Takumi Abe, Zen-ichi Yamamoto, Hirotomo Noda,
Takahiro Iwata, Shin-ya Murakami, Takehiko Satoh, Tetsuya Fukuhara, Kazunori Ogohara, Ko-ichiro Sugiyama,
Hiroki Kashimura, Shoko Ohtsuki, Seiko Takagi, Yukio Yamamoto, Naru Hirata, George L. Hashimoto,
Manabu Yamada, Makoto Suzuki, Nobuaki Ishii, Tomoko Hayashiyama, Yeon Joo Lee and Masato Nakamura
Earth, Planets and Space 2017, 69:137 DOI:10.1186/s40623-017-0722-3
Received: 4 July 2017, Accepted: 20 September 2017, Published: 3 October 2017

Abstract
After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation
experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stableoscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are theatmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles weresuccessfully obtained down to ~ 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall
structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent featuresare seen within and above the clouds, which is located around 48–70 km altitude. The H2SO4 vapor density roughly follows the saturation
curve at cloud heights, suggesting equilibrium with cloud particles. The
ionospheric electron density profiles are also successfully retrieved,
showing distinct local time dependence. Akatsuki RS mainly probes the
low and middle latitude regions thanks to the near-equatorial orbit incontrast to the previous radio occultation experiments using polar
orbiters. Studies based on combined analyses of RS and optical imaging
data are ongoing.
90

Altitude (km)

80
70
60
50
40

Keywords: Venus, Radio occultation, Akatsuki

150

200

250

300

350

400

450

Temperature (K)Graphical abstract*Corresponding author: Takeshi Imamura, t_imamura@edu.k.u‑tokyo.ac.jp

FULL PAPER

Open Access

Absolute calibration of brightness temperature of the Venus disk
observed by the Longwave Infrared Camera onboard Akatsuki
Tetsuya Fukuhara*, Makoto Taguchi, Takeshi Imamura, Akane Hayashitani, Takeru Yamada, Masahiko Futaguchi,
Toru Kouyama, Takao M. Sato, Mao Takamura, Naomoto Iwagami, Masato Nakamura, Makoto Suzuki,
Munetaka Ueno, George L. Hashimoto, Mitsuteru Sato, Seiko Takagi, Atsushi Yamazaki, Manabu Yamada,Shin-ya Murakami, Yukio Yamamoto, Kazunori Ogohara, Hiroki Ando, Ko-ichiro Sugiyama, Hiroki Kashimura,Shoko Ohtsuki, Nobuaki Ishii, Takumi Abe, Takehiko Satoh, Chikako Hirose and Naru Hirata
Earth, Planets and Space 2017, 69:141 DOI:10.1186/s40623-017-0727-y
Received: 26 July 2017, Accepted: 29 September 2017, Published: 6 October 2017

Abstract
The Venus Climate Orbiter Akatsuki arrived at Venus in December 2015, and the Longwave Infrared Camera (LIR) onboard the spacecraft
started making observations. LIR has acquired more than 8000 images during the first two Venusian years since orbit insertion without any
serious faults. However, brightness temperature derived from LIR images contained an unexpected bias that related not to natural
phenomena but to a thermal condition of the instrument. The bias could be partially eliminated by keeping the power supply unit for LIR
always active, while the residual bias was simply correlated with the baffle
temperature. Therefore, deep-space images were acquired at different baffle
temperatures on orbit, and a reference table for eliminating the bias fromimages was prepared. In the corrected images, the brightness temperature was~ 230 K at the center of the Venus disk, where the effect of limb darkening is
negligible. The result is independent of the baffle temperature and consistent
with the results of previous studies. Later, a laboratory experiment with theproto model of LIR showed that when the germanium (Ge) lens was heated, its
actual temperature was slightly higher than the temperature measured by athermal sensor attached to the lens holder. The experiment confirmed thatb The same image correcteda Original LIR image representing
transitory baffle heating accounted for the background bias found in thefor the background bias .
the brightness temperature of thebrightness temperature observed by LIR.
Venus disk. It was acquired on 19
October 2016, with the baffle tem‑
perature being 334.6 K.

Keywords: Venus, Thermal infrared, Akatsuki

Graphical abstract*Corresponding author: Tetsuya Fukuhara, tetsuyaf@rikkyo.ac.jp
5Akatsuki at Venus: The First Year of Scientific Operation

FULL PAPER

Open Access

Effects of variation in coagulation and photochemistryparameters on the particle size distributions in the Venus
cloudsKevin McGouldrick
Earth, Planets and Space 2017, 69:161 DOI:10.1186/s40623-017-0744-x
Received: 4 July 2017, Accepted: 14 November 2017, Published: 1 December 2017

Abstract
This paper explores the effects that variation in the coalescence efficiency of the Venus cloud particles can have on the
structure of the Venus cloud. It is motivated by the acknowledgment of uncertainties in the measured parameters—and theassumptions made to account for them—that define our present knowledge of the particle characteristics. Specifically, we
explore the consequence of allowing the coalescence efficiency of supercooled sulfuric acid in the upper clouds to tend tozero. This produces a cloud that occasionally exhibits an enhancement of small particles at altitude (similar to the upper hazes
observed by Pioneer Venus and subsequently shown to be somewhat transient). This simulated cloud occasionally exhibits a
rapid growth of particle size near cloud base, exhibiting characteristics similar to those seen in the controversial Mode 3
particles. These results demonstrate that a subset of the variations observed as near-infrared opacity variations in the lower
and middle clouds of Venus can be explained by microphysical, in
addition to dynamical, variations. Furthermore, the existence of a
population of particles exhibiting less efficient coalescence efficiencies
would support the likelihood of conditions suitable for charge
exchange, hence lightning, in the Venus clouds. We recommend futurelaboratory studies on the coalescence properties of sulfuric acid under
the range of conditions experienced in the Venus clouds. We also
recommend future in situ measurements to better characterize theproperties of the cloud particles themselves, especially compositionand particle habits (shapes).

Keywords: Venus, Atmosphere, Clouds, Meteorology, Dynamics

Graphical abstract

Corresponding author: Kevin McGouldrick, kevin.mcgouldrick@lasp.colorado.edu

FULL PAPER

Open Access

Overview of Akatsuki data products: definition of data levels, method
and accuracy of geometric correction
Kazunori Ogohara*, Masahiro Takagi, Shin-ya Murakami, Takeshi Horinouchi, Manabu Yamada, Toru Kouyama,
George L. Hashimoto, Takeshi Imamura, Yukio Yamamoto, Hiroki Kashimura, Naru Hirata, Naoki Sato,
Atsushi Yamazaki, Takehiko Satoh, Naomoto Iwagami, Makoto Taguchi, Shigeto Watanabe, Takao M. Sato,
Shoko Ohtsuki, Tetsuya Fukuhara, Masahiko Futaguchi, Takeshi Sakanoi, Shingo Kameda, Ko-ichiro Sugiyama,
Hiroki Ando, Yeon Joo Lee, Masato Nakamura, Makoto Suzuki, Chikako Hirose, Nobuaki Ishii and Takumi Abe
Earth, Planets and Space 2017, 69:167 DOI:10.1186/s40623-017-0749-5
Received: 28 July 2017, Accepted: 27 November 2017, Published: 12 December 2017

Abstract
We provide an overview of data products from observations by the Japanese Venus Climate Orbiter, Akatsuki, and describe thedefinition and content of each data-processing level. Levels 1 and 2 consist of non-calibrated and calibrated radiance (orbrightness temperature), respectively, as well as geometry information (e.g., illumination angles). Level 3 data are global-griddata in the regular longitude–latitude coordinate system, produced from the contents of Level 2. Non-negligible errors innavigational data and instrumental alignment can result in serious errors in the geometry calculations. Such errors cause
mismapping of the data and lead to inconsistencies between radiances and illumination angles, along with errors in cloudmotion vectors. Thus, we carefully correct the boresight pointing of each camera by fitting an ellipse to the observed Venusianlimb to provide improved longitude–latitude maps for Level 3 products, if possible. The accuracy of the pointing correction is
also estimated statistically by simulating observed limb
distributions. The results show that our algorithm
successfully corrects instrumental pointing and will
enable a variety of studies on the Venusian atmosphereusing Akatsuki data.

Keywords: Venus, Data product, Pointing correction,
Ellipse-fitting technique

Graphical abstract*Corresponding author: Kazunori Ogohara, ogohara.k@e.usp.ac.jp
6Earth, Planets and Space

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Open Access

Initial products of Akatsuki 1-μm camera
Naomoto Iwagami*, Takeshi Sakanoi, George L. Hashimoto, Kenta Sawai, Shoko Ohtsuki, Seiko Takagi,
Kazunori Uemizu, Munetaka Ueno, Shingo Kameda, Shin-ya Murakami, Masato Nakamura, Nobuaki Ishii,
Takumi Abe, Takehiko Satoh, Takeshi Imamura, Chikako Hirose, Makoto Suzuki, Naru Hirata, Atsushi Yamazaki,
Takao M. Sato, Manabu Yamada, Yukio Yamamoto, Tetsuya Fukuhara, Kazunori Ogohara, Hiroki Ando,
Ko-ichiro Sugiyama, Hiroki Kashimura and Toru Kouyama
Earth, Planets and Space 2018, 70:6 DOI:10.1186/s40623-017-0773-5
Received: 9 July 2017, Accepted: 25 December 2017, Published: 11 January 2018

Abstract
The status and initial products of the 1-μm camera onboard the Akatsuki mission to Venus are presented. After the successful
retrial of Venus’ orbit insertion on Dec. 2015 (5 years after the failure in Dec. 2010), and after a long cruise under intense radiation,
damage in the detector seems small and fortunately insignificant in the final quality of the images. More than 600 dayside
images have been obtained since the beginning of regular operation on Apr. 2016 although nightside images are less numerous(about 150 in total at three wavelengths) due to the light scattered from the bright dayside. However, data acquisition stopped
after December 07, 2016, due to malfunction of the electronics and has not been resumed since then. The 0.90-µm dayside
images are of sufficient quality for the cloud-tracking procedure to retrieve wind field in the cloud region. The results appear tobe similar to those reported by previous 1-μm imaging by Galileo andVenus Express. The representative altitude sampled for such dayside
images is estimated to be 51–55 km. Also, the quality of the nightside
1.01-µm images is sufficient for a search for active volcanism, sinceinterference due to cloud inhomogeneity appears to be insignificant. The
quality of the 0.97-µm images may be insufficient to achieve the expected
spatial resolution for the near-surface H2O mixing ratio retrievals.

Keywords: Venus, Infrared, Dayside cloud, Nightside surface
Graphical abstract*Corresponding author: Naomoto Iwagami, iwagamina@hotmail.co.jp

FULL PAPER

Open Access

Mean winds at the cloud top of Venus obtained fromtwo-wavelength UV imaging by Akatsuki
Takeshi Horinouchi*, Toru Kouyama, Yeon Joo Lee, Shin-ya Murakami, Kazunori Ogohara, Masahiro Takagi,
Takeshi Imamura, Kensuke Nakajima, Javier Peralta, Atsushi Yamazaki, Manabu Yamada and Shigeto Watanabe
Earth, Planets and Space 2018, 70:10 DOI:10.1186/s40623-017-0775-3
Received: 15 July 2017, Accepted: 25 December 2017, Published: 15 January 2018

Abstract
Venus is covered with thick clouds. Ultraviolet (UV) images at 0.3–0.4 microns show detailed cloud features at the cloud-top level at about 70 km,
which are created by an unknown UV-absorbing substance. Images acquired in this wavelength range have traditionally been used to measurewinds at the cloud top. In this study, we report low-latitude winds obtained from the images taken by the UV imager, UVI, onboard the Akatsuki
orbiter from December 2015 to March 2017. UVI provides images with two filters centered at 365 and 283 nm. While the 365-nm images enable
continuation of traditional Venus observations, the 283-nm images visualize cloud features at an SO2 absorption band, which is novel. We used a
sophisticated automated cloud-tracking method and thorough quality control to estimate winds with high precision. Horizontal winds obtained
from the 283-nm images are generally similar to those from the 365-nm images, but in many cases, westward winds from the former are fasterthan the latter by a few m/s. From previous studies, one can argue that the 283-nm images likely reflect cloud features at higher altitude than the
365-nm images. If this is the case, the superrotation of the Venusian atmosphere generally increases with height at the cloud-top level, where it
has been thought to roughly peak. The mean winds obtained from the 365-nm images exhibit local time dependence consistent with known tidal
features. Mean zonal winds exhibit asymmetry with respect to the equator in the latter half of the analysis period, significantly at 365 nm andweakly at 283 nm. This contrast indicates that the relative altitude may vary with time and latitude, and so are the observed altitudes. In contrast,
mean meridional winds do not exhibit much long-term variability. A previous study
suggested that the geographic distribution of temporal mean zonal winds obtained fromUV images from the Venus Express orbiter during 2006–2012 can be interpreted as forced
by topographically induced stationary gravity waves. However, the geographic distributionof temporal mean zonal winds we obtained is not consistent with that distribution, whichsuggests that the distribution may not be persistent.

Keywords: Venus, Planetary atmosphere, Planetary climatology, Cloud tracking,Image velocimetry, Superrotation, Aerosol, Wind shear, Cloud motion
vector, SO2*Corresponding author: Takeshi Horinouchi, horinout@ees.hokudai.ac.jp
7Graphical abstract

Akatsuki at Venus: The First Year of Scientific Operation

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Open Access

Ultraviolet imager on Venus orbiter Akatsuki and its initial results
Atsushi Yamazaki*, Manabu Yamada, Yeon Joo Lee, Shigeto Watanabe, Takeshi Horinouchi, Shin-ya Murakami,
Toru Kouyama, Kazunori Ogohara, Takeshi Imamura, Takao M. Sato, Yukio Yamamoto, Tetsuya Fukuhara,
Hiroki Ando, Ko-ichiro Sugiyama, Seiko Takagi, Hiroki Kashimura, Shoko Ohtsuki, Naru Hirata,
George L. Hashimoto, Makoto Suzuki, Chikako Hirose, Munetaka Ueno, Takehiko Satoh, Takumi Abe,
Nobuaki Ishii and Masato Nakamura
Earth, Planets and Space 2018, 70:23 DOI:10.1186/s40623-017-0772-6
Received: 14 July 2017, Accepted: 25 December 2017, Published: 12 February 2018

Abstract
The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes
ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the 283
and 365 nm wavelengths. There are absorption bands of SO2 and unknown absorbers in these wavelength regions. The UV
images provide the spatial distribution of SO2 and the unknown absorber around cloud top altitudes. The images also allow us
to understand the cloud top morphologies and haze properties. Nominal sequential images with 2-h intervals are used tounderstand the dynamics of the Venusian atmosphere by estimating the wind vectors at the cloud top altitude, as well as themass transportation of UV absorbers. The UVI is equipped with off-axial
catadioptric optics, two bandpass filters, a diffuser installed in a filter wheel
moving with a step motor, and a high sensitivity charge-coupled device with
UV coating. The UVI images have spatial resolutions ranging from 200 m to 86
km at sub-spacecraft points. The UVI has been kept in good condition during
the extended interplanetary cruise by carefully designed operations that havemaintained its temperature maintenance and avoided solar radiation damage.
The images have signal-to-noise ratios of over 100 after onboard desmear
processing.

Keywords: Venus orbiter Akatsuki, Ultraviolet imager (UVI), UVI performance,
UV images of Venus at the cloud top altitude, Initial results ofcloud tracking‑‑

Fig. 1*Corresponding author: Atsushi Yamazaki, yamazaki@stp.isas.jaxa.jp

FULL PAPER

Open Access

Venus looks different from day to night across wavelengths:
morphology from Akatsuki multispectral images
Sanjay S. Limaye*, Shigeto Watanabe, Atsushi Yamazaki, Manabu Yamada, Takehiko Satoh, Takao M. Sato,
Masato Nakamura, Makoto Taguchi, Tetsuya Fukuhara, Takeshi Imamura, Toru Kouyama, Yeon Joo Lee,
Takeshi Horinouchi, Javier Peralta, Naomoto Iwagami, George L. Hashimoto, Seiko Takagi, Shoko Ohtsuki,
Shin-ya Murakami, Yukio Yamamoto, Kazunori Ogohara, Hiroki Ando, Ko-ichiro Sugiyama, Nobuaki Ishii,
Takumi Abe, Chikako Hirose, Makoto Suzuki, Naru Hirata, Eliot F. Young and Adriana C. Ocampo
Earth, Planets and Space 2018, 70:24 DOI:10.1186/s40623-018-0789-5
Received: 9 August 2017, Accepted: 24 January 2018, Published: 12 February 2018

Abstract
Since insertion into orbit on December 7, 2015, the Akatsuki orbiter has returned global images of Venus from its four imaging cameras at elevendiscrete wavelengths from ultraviolet (283 and 365 nm) and near infrared (0.9–2.3 µm), to the thermal infrared (8–12 µm) from a near-equatorial
orbit. The Venus Express and Pioneer Venus Orbiter missions have also monitored the planet for long periods but from polar or near-polar orbits.
The wavelength coverage and views of the planet also differ for all three missions. In reflected light, the images reveal features seen near the
cloud tops (~ 70 km altitude), whereas in the near-infrared images of the nightside, features seen are at mid- to lower cloud levels (~ 48–60 km
altitude). The dayside cloud cover imaged at the ultraviolet wavelengths shows morphologies similar to what wasobserved from Mariner 10, Pioneer Venus, Galileo, Venus Express and MESSENGER. The daytime images at 0.9 and 2.02
µm also reveal some interesting features which bear similarity to the ultraviolet images. The nighttime images at 1.74,
2.26 and 2.32 µm and at 8–12 µm reveal features not seen before and show new details of the nightside including
narrow wavy ribbons, curved string-like features, long-scale waves, long dark streaks, isolated bright spots, sharpboundaries and even mesoscale vortices. Some features previously seen such as circum-equatorial belts (CEBs) andoccasional areal brightenings at ultraviolet (seen in Venus Express observations) of the cloud cover at ultraviolet
wavelengths have not been observed thus far. Evidence for the hemispheric vortex organization of the global
circulation can be seen at all wavelengths on the day- and nightsides. Akatsuki images reveal new and puzzling
morphology of the complex nightside cloud cover. The cloud morphologies provide some clues to the processes
occurring in the atmosphere and are thus, a key diagnostic tool when quantitative dynamical analysis is not feasible
due to insufficient information.

Keywords: Venus clouds, Morphology, Day, Night, Ultraviolet, Near infrared*Corresponding author: Sanjay S. Limaye, Sanjay.Limaye@ssec.wisc.edu
8Graphical abstract

Earth, Planets and Space

FULL PAPER

Open Access

Initiation of a lightning search using the lightning and airglow camera
onboard the Venus orbiter Akatsuki
Yukihiro Takahashi*, Mitsuteru Sato, Masataka Imai, Ralph Lorenz, Yoav Yair, Karen Aplin, Georg Fischer,
Masato Nakamura, Nobuaki Ishii, Takumi Abe, Takehiko Satoh, Takeshi Imamura, Chikako Hirose,
Makoto Suzuki, George L. Hashimoto, Naru Hirata, Atsushi Yamazaki, Takao M. Sato, Manabu Yamada,Shin-ya Murakami, Yukio Yamamoto, Tetsuya Fukuhara, Kazunori Ogohara, Hiroki Ando, Ko-ichiro Sugiyama,
Hiroki Kashimura and Shoko Ohtsuki
Earth, Planets and Space 2018, 70:88 DOI:10.1186/s40623-018-0836-2
Received: 1 July 2017, Accepted: 14 April 2018, Published: 25 May 2018

Abstract
The existence of lightning discharges in the Venus atmosphere has been controversial for more than 30 years, with many
positive and negative reports published. The lightning and airglow camera (LAC) onboard the Venus orbiter, Akatsuki, wasdesigned to observe the light curve of possible flashes at a sufficiently high sampling rate to discriminate lightning from othersources and can thereby perform a more definitive search for optical emissions. Akatsuki arrived at Venus during December
2016, 5 years following its launch. The initial operations of LAC through November
2016 have included a progressive increase in the high voltage applied to theavalanche photodiode detector. LAC began lightning survey observations in
December 2016. It was confirmed that the operational high voltage was achieved andthat the triggering system functions correctly. LAC lightning search observations areplanned to continue for several years.

Keywords: Venus, Lightning, Flash, Akatsuki, Lightning and airglow camera

Graphical abstract*Corresponding author: Yukihiro Takahashi, yukihiro@sci.hokudai.ac.jp

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Contents
Special issue “Akatsuki at Venus: The First Year of Scientific Operation”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masato Nakamura, Dmitri Titov, Kevin McGouldrick, Pierre Drossart, Jean-Loup Bertaux and Huixin Liu
AKATSUKI returns to Venus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masato Nakamura, Takeshi Imamura, Nobuaki Ishii, Takumi Abe, Yasuhiro Kawakatsu, Chikako Hirose,

Takehiko Satoh, Makoto Suzuki, Munetaka Ueno, Atsushi Yamazaki, Naomoto Iwagami, Shigeto Watanabe,
Makoto Taguchi, Tetsuya Fukuhara, Yukihiro Takahashi, Manabu Yamada, Masataka Imai, Shoko Ohtsuki,
Kazunori Uemizu, George L. Hashimoto, Masahiro Takagi, Yoshihisa Matsuda, Kazunori Ogohara, Naoki Sato,
Yasumasa Kasaba, Toru Kouyama, Naru Hirata, Ryosuke Nakamura, Yukio Yamamoto, Takeshi Horinouchi,
Masaru Yamamoto, Yoshi-Yuki Hayashi, Hiroki Kashimura, Ko-ichiro Sugiyama, Takeshi Sakanoi, Hiroki Ando,
Shin-ya Murakami, Takao M. Sato, Seiko Takagi, Kensuke Nakajima, Javier Peralta, Yeon Joo Lee, Junichi Nakatsuka,
Tsutomu Ichikawa, Kozaburo Inoue, Tomoaki Toda, Hiroyuki Toyota, Sumitaka Tachikawa,
Shinichiro Narita, Tomoko Hayashiyama, Akiko Hasegawa and Yukio Kamata

1

4

Performance of Akatsuki/IR2 in Venus orbit: the first year
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takehiko Satoh, Takao M. Sato, Masato Nakamura, Yasumasa Kasaba, Munetaka Ueno, Makoto Suzuki,

George L. Hashimoto, Takeshi Horinouchi, Takeshi Imamura, Atsushi Yamazaki, Takayuki Enomoto, Yuri Sakurai,
Kosuke Takami, Kenta Sawai, Takashi Nakakushi, Takumi Abe, Nobuaki Ishii, Chikako Hirose, Naru Hirata,
Manabu Yamada, Shin-ya Murakami, Yukio Yamamoto, Tetsuya Fukuhara, Kazunori Ogohara, Hiroki Ando,
Ko-ichiro Sugiyama, Hiroki Kashimura and Shoko Ohtsuki

4

Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki. . . . . . . . . . . . . . . . . . . . . Takeshi Imamura, Hiroki Ando, Silvia Tellmann, Martin Pätzold, Bernd Häusler, Atsushi Yamazaki, Takao M. Sato,

Katsuyuki Noguchi, Yoshifumi Futaana, Janusz Oschlisniok, Sanjay Limaye, R. K. Choudhary, Yasuhiro Murata,
Hiroshi Takeuchi, Chikako Hirose, Tsutomu Ichikawa, Tomoaki Toda, Atsushi Tomiki, Takumi Abe,
Zen-ichi Yamamoto, Hirotomo Noda, Takahiro Iwata, Shin-ya Murakami, Takehiko Satoh,Tetsuya Fukuhara, Kazunori Ogohara, Ko-ichiro Sugiyama, Hiroki Kashimura, Shoko Ohtsuki,
Seiko Takagi, Yukio Yamamoto, Naru Hirata, George L. Hashimoto, Manabu Yamada,
Makoto Suzuki, Nobuaki Ishii, Tomoko Hayashiyama, Yeon Joo Lee and Masato Nakamura

5

Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera
onboard Akatsuki. . . . . . . . . . . . . . . . . . . . . . . Tetsuya Fukuhara, Makoto Taguchi, Takeshi Imamura, Akane Hayashitani, Takeru Yamada,
Masahiko Futaguchi, Toru Kouyama, Takao M. Sato, Mao Takamura, Naomoto Iwagami, Masato Nakamura,
Makoto Suzuki, Munetaka Ueno, George L. Hashimoto, Mitsuteru Sato, Seiko Takagi, Atsushi Yamazaki, Manabu Yamada,Shin-ya Murakami, Yukio Yamamoto, Kazunori Ogohara, Hiroki Ando, Ko-ichiro Sugiyama, Hiroki Kashimura,Shoko Ohtsuki, Nobuaki Ishii, Takumi Abe, Takehiko Satoh, Chikako Hirose and Naru Hirata

Effects of variation in coagulation and photochemistry parameters on the particle size distributions in the Venus clouds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kevin McGouldrick
Overview of Akatsuki data products: definition of data levels, method and accuracy of geometric correction. . . . . . . . . . . . . . . . . . . . . . . Kazunori Ogohara, Masahiro Takagi, Shin-ya Murakami, Takeshi Horinouchi, Manabu Yamada, Toru Kouyama,
George L. Hashimoto, Takeshi Imamura, Yukio Yamamoto, Hiroki Kashimura, Naru Hirata, Naoki Sato,
Atsushi Yamazaki, Takehiko Satoh, Naomoto Iwagami, Makoto Taguchi, Shigeto Watanabe,
Takao M. Sato, Shoko Ohtsuki, Tetsuya Fukuhara, Masahiko Futaguchi, Takeshi Sakanoi,
Shingo Kameda, Ko-ichiro Sugiyama, Hiroki Ando, Yeon Joo Lee, Masato Nakamura,
Makoto Suzuki, Chikako Hirose, Nobuaki Ishii and Takumi Abe

566

Initial products of Akatsuki 1-μm camera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Naomoto Iwagami, Takeshi Sakanoi, George L. Hashimoto, Kenta Sawai, Shoko Ohtsuki, Seiko Takagi,

Kazunori Uemizu, Munetaka Ueno, Shingo Kameda, Shin-ya Murakami, Masato Nakamura, Nobuaki Ishii,
Takumi Abe, Takehiko Satoh, Takeshi Imamura, Chikako Hirose, Makoto Suzuki, Naru Hirata, Atsushi Yamazaki,
Takao M. Sato, Manabu Yamada, Yukio Yamamoto, Tetsuya Fukuhara, Kazunori Ogohara,
Hiroki Ando, Ko-ichiro Sugiyama, Hiroki Kashimura and Toru Kouyama

Mean winds at the cloud top of Venus obtained from two-wavelength UV imaging by Akatsuki. . . . . . . . . . . . . . . . . . . . . . . . . . . Takeshi Horinouchi, Toru Kouyama, Yeon Joo Lee, Shin-ya Murakami, Kazunori Ogohara, Masahiro Takagi,
Takeshi Imamura, Kensuke Nakajima, Javier Peralta, Atsushi Yamazaki, Manabu Yamada and Shigeto Watanabe

77

Ultraviolet imager on Venus orbiter Akatsuki and its initial results. . . . . . . . . . . . . . . . . . . . . . . Atsushi Yamazaki, Manabu Yamada, Yeon Joo Lee, Shigeto Watanabe, Takeshi Horinouchi, Shin-ya Murakami,
Toru Kouyama, Kazunori Ogohara, Takeshi Imamura, Takao M. Sato, Yukio Yamamoto, Tetsuya Fukuhara,
Hiroki Ando, Ko-ichiro Sugiyama, Seiko Takagi, Hiroki Kashimura, Shoko Ohtsuki, Naru Hirata,
George L. Hashimoto, Makoto Suzuki, Chikako Hirose, Munetaka Ueno, Takehiko Satoh,
Takumi Abe, Nobuaki Ishii and Masato Nakamura

8

Venus looks different from day to night across wavelengths: morphology from Akatsuki multispectral images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sanjay S. Limaye, Shigeto Watanabe, Atsushi Yamazaki, Manabu Yamada, Takehiko Satoh, Takao M. Sato,
Masato Nakamura, Makoto Taguchi, Tetsuya Fukuhara, Takeshi Imamura, Toru Kouyama, Yeon Joo Lee,
Takeshi Horinouchi, Javier Peralta, Naomoto Iwagami, George L. Hashimoto, Seiko Takagi, Shoko Ohtsuki,
Shin-ya Murakami, Yukio Yamamoto, Kazunori Ogohara, Hiroki Ando, Ko-ichiro Sugiyama, Nobuaki Ishii,
Takumi Abe, Chikako Hirose, Makoto Suzuki, Naru Hirata, Eliot F. Young and Adriana C. Ocampo

8

Initiation of a lightning search using the lightning and airglow camera onboard the Venus orbiter Akatsuki. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yukihiro Takahashi, Mitsuteru Sato, Masataka Imai, Ralph Lorenz, Yoav Yair, Karen Aplin, Georg Fischer,

Masato Nakamura, Nobuaki Ishii, Takumi Abe, Takehiko Satoh, Takeshi Imamura, Chikako Hirose,
Makoto Suzuki, George L. Hashimoto, Naru Hirata, Atsushi Yamazaki, Takao M. Sato, Manabu Yamada,Shin-ya Murakami, Yukio Yamamoto, Tetsuya Fukuhara, Kazunori Ogohara, Hiroki Ando,
Ko-ichiro Sugiyama, Hiroki Kashimura and Shoko Ohtsuki

9